Torque measurement apparatus

ABSTRACT

Torque in a shaft ( 61 ) is detected by means of non-contacting sensors ( 23, 24 ) sensing a torque-dependent magnetic field emanated by an integral transducer region ( 64, 32 ) of the shaft ( 61 ) that is circumferentially or longitudinally magnetized. The shaft ( 61 ) is driven by a motor ( 63 ) and subject to a longitudinal magnetic field ( 60 ) which acts on interference field. In one implementation of the invention coils (L 1 , L 2 : L 3 , L 4 ) are energized to provide a counteracting magnetic field to compensate the interference field ( 60 ).

FIELD OF THE INVENTION

The present invention relates to the measurement of torque generated ina drive shaft. More particularly, it concerns the non-contactingmeasurement of such torque using magnetised transducers and seeks tocompensate for, eliminate or avoid the effects of interfering magneticfields.

BACKGROUND TO THE INVENTION

There have been prior proposals to use magnetised transducer elementsfor torque measurement, the transducer elements being a ring attached toa torqued shaft or the shaft itself. In this connection reference ismade to U.S. Pat. Nos. 5,351,555, 5,465,627 and 5,520,059 and topublished PCT Applications WO99/21150, WO99/21151 and WO99/56099. Inthese specifications the ring or shaft is of magnetoelastic materialcircumferentially magnetised, that is the magnetisation forms a closedloop around the shaft. While such transducer elements are usable in thepractice of this invention, other patterns of magnetisation are usableand do not necessarily rely on magnetoelasticity, and other shapes oftransducer element may be employed. One other pattern of magnetisationwhich may be employed in the practice of this invention longitudinalmagnetisation of the transducer region. One form of longitudinalmagnetisation is disclosed in International patent applicationPCT/GB00/03119 filed 14th Aug. 2000 and published under the numberWO/01/13081 A1.

It is a feature of transducers systems employing magnetised transducerelements of the kind outlined above, that the torque dependent fieldcomponent provided by the transducer element can be sensed by one ormore sensors adjacent to but not in contact with the transducerelements. Non-contacting sensor arrangements are of particular value intorque measurement on rotating shafts.

The above techniques are based on magnetic principles and therefore canbe affected by other interfering magnetic fields, like the earth'smagnetic field or fields generated by electric motors for example. Insome environments where it is desirable to measure shaft torque, verystrong magnetic fields may be present, particularly in the longitudinalaxis of the sensing system. A typical application of this nature is theextended axis of an electric motor having a shaft projecting from themotor.

SUMMARY OF THE INVENTION

The present invention is predicated on a number of different approaches.A first may be broadly expressed as compensating or counteracting aninterfering magnetic field. A second may be broadly expressed as aselective signal approach, particularly by introducing a frequencyselective element into the torque-dependent magnetic flux to be measuredthat enables it to be distinguished from signals due to an interferingfield. A third approach is to turn the “interfering” magnetic field touse and employ it as a source field from which to obtain atorque-dependent component. A fourth approach is a new way of measuringtorque to which a frequency selective element may be applied. It ispossible to use combinations of these approaches, particularly incombining the first approach with the second or third.

One implementation of the present invention according to the firstapproach above-mentioned provides a torque transducer for measuringtorque in a rotating shaft of the kind having a transducer region inwhich a magnetic transducer field is established and at least onenon-contacting sensor adjacent the transducer region to develop atorque-dependent signal, wherein in operation the shaft is subject tolongitudinal flux generated by means external to the transducer region,characterised by means magnetically coupled to said shaft to generate acompensating flux to counteract said longitudinal flux at the transducerregion.

Preferably, the means coupled to the shaft for generating thecompensating flux comprises at least one current-carrying coil about theshaft. It may comprise a pair of axially spaced coils between which thetransducer region is situate. In the alternative or additionally, amagnetic structure may also be provided which has poles axially spacedalong the shaft and at least one coil is wound about said magneticstructure.

An implementation of the invention according to the third approachabove-mentioned provides a torque transducer for measuring the torque ina rotating shaft which, in operation, has a longitudinal field extendingtherealong, wherein at least one sensor is placed in non-contactingfashion adjacent a portion of the shaft to sense and provide a signaldependent on a transverse component of flux arising from thelongitudinal flux in response to the torque in the shaft. Morespecifically a transverse component is transverse to the axis ofrotation and at the surface of the shaft portion is usually detected asa component in the circumferential or tangential direction. In thepreferred embodiment, at least one further non-contacting sensor ismounted to sense the longitudinal flux to provide a reference signaldependent thereon against which to measure the transverse component foruse in obtaining a value for the torque in the shaft.

In yet another implementation of the invention, this time in accord withthe second, selective signal approach above-mentioned, a torquetransducer for measuring the torque in a rotating shaft includes aportion or region of the shaft which acts as a transducer element andwhich is disposed between a pair of coils encircling the shaft andconnected to induce a longitudinal magnetic field through the transducerregion upon energisation of the coils. The coils are connected to an ACsource, preferably a pulsed source, operating at a selected frequency sothat the transducer region is subject to a magnetic field of alternatingpolarity. A sensor arrangement is responsive to a torque-dependentcomponent of the alternating magnetic field and provides an AC outputprocessed in a frequency-selective manner linked to the source frequencyto extract the wanted component from any other noise (DC or AC) that maybe present. The frequency-selective processing may be by way of ahardware or software implemented filter operating at the selectedfrequency linked with the AC source to synchronize the filter frequencyto the source frequency. A synchronous detection scheme can be useddetecting the sensor output signal with the aid of the AC source outputto provide an inherent filtering operation.

According to another implementation, a transducer assembly formeasuring, preferably in a non-contacting fashion, torque in a rotatingshaft, comprises an erase head for cleaning a zone of the shaft as itrotates, a write head downstream of the erase head in the direction ofrotation to write a magnetic track onto the cleaned zone, said trackhaving a given width, a pair of read heads spaced in an axial directionto respond to the magnetic track, said read heads being disposed on,toward or adjacent opposite sides of the track to generate respectivesignals, and differential means responsive to said respective signals toprovide a signal dependent on torque in the shaft. It is preferred toenergise the write head with an AC signal, preferably a pulsed signal,to detect the AC outputs of the read heads derived from the AC modulatedtrack. The detection can be done in a frequency-selective manner toenhance discrimination from other signal fields that may be present. Itis preferred that the write head be oriented with the head gap in thecircumferential or tangential direction.

Aspects and features of this invention are set forth in the claimsfollowing this description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example andwith reference to the accompanying drawings wherein:

FIGS. 1 to 3 illustrate measurement of shaft torque usingcircumferential magnetisation;

FIGS. 4 and 5 illustrate measurement of shaft torque using longitudinalmagnetisation;

FIG. 6 shows the longitudinal magnetic flux developed in the shaft of atypical electric motor;

FIGS. 7 and 8a show apparatus for cancelling an interfering magneticfield generated by an electric motor according to a first embodiment ofthe invention;

FIG. 8b is an end view of the shaft shown in FIG. 8a;

FIGS. 9a and 9 b show side and end views of a shielded and activelycompensated transducer in accordance with a second embodiment of theinvention;

FIGS. 10a and 10 b show side and end views, respectively, of apparatusfor measuring shaft torque using a magnetic field in the shaft accordingto a third embodiment of the invention;

FIG. 11 shows a deflected magnetic field in the shaft of FIG. 10a;

FIG. 12 shows an arrangement for eliminating the effects of aninterfering magnetic field by using a transducer system operating at aselected frequency in accordance with a fourth embodiment of theinvention; and

FIGS. 13a and 13 b show side and end views of an arrangement formeasuring shaft torque using magnetic erase, read and write headsadjacent a shaft, according to a fifth embodiment of the invention.

FIGS. 1 to 3 illustrate detection of shaft torque using the technique ofcircumferential magnetisation referred to above. FIG. 1 illustrates thecircumferential field, indicated by arrow, 2 under no torque conditionin a transducer region 3 of a shaft 4 rotatable about axis A—A. FIG. 2illustrates the closed loop nature of the field in a surface adjacentzone of the region shaft 3. The region 3 exhibits magnetoelasticity.Under “no torque” the circumferential field 2 in region 3 is entirelycontained in the region 4: there is no external fringe field. Undertorque, as seen in FIG. 3 the field 2 is skewed to produce anaxially-directed North-South (NS) magnetisation whose polarity andmagnitude are dependent on the direction (clockwise or counterclockwise)of the torque and its magnitude. The axial magnetisation emanates anexternal fringe field dependent on torque which is measurable by asensor 7, or more usually by a sensor arrangement comprising a pluralityof sensors. The sensor(s) may be of the Hall-effect or magnetoresistivetype but preferably are of the saturating core type connected in acircuit such as disclosed in published PCT application WO98/52063.

FIGS. 4 and 5 demonstrate detection of shaft torque using longitudinalmagnetisation of a region 3′ of the shaft 4. The region 3′ is ofmagnetic material. The longitudinal field 8 lies along the shaft in asurface adjacent annulus forming a torus of magnetic flux which closesmainly in an inner zone of the regions 31 to form a closed toroidalloop. The surface field all lies in the same direction. There is a smallquiescent longitudinal fringe field 10 that leaks from the shaft as seenin FIG. 4. In the form of longitudinal magnetisation being discussed,under torque, the field 2′ skews (FIG. 5) as indicated by the dashedarrows 2″ and produces a small transverse or circumferential componentdetectable by sensor 22: the longitudinal component is detectable bysensor 21. The sensors of the types already mentioned have directionalresponses and are oriented to be responsive to the desired fieldcomponent.

Further information on the form of longitudinal magnetisation discussedabove and the means of producing it is found in International patentapplication PCT/GB00/03119 (WO 01/13081 A1) which is incorporated hereinby reference.

Attention can now be given to problems which arise when the shaft 4 isdriven, and thus put under torque by a machine such as an electricmotor.

An electric motor 63 is diagrammatically shown in FIG. 6. It has anintegral output shaft 61 which is susceptible to providing a path formagnetic forces generated by the motor during its operation. Dependingon the specific design of the motor and of the shaft 61 driven therebysome magnetic field can exit the motor assembly (unintentionally orinadvertently) through the drive shaft 61 of the motor 63 as indicatedby arrows 60. This assumes the shaft is of a ferromagnetic material andis capable of supporting a transducer region of the kind described abovein an integral portion of the shaft.

When trying to measure the mechanical torque generated by the electricmotor 63 in the shaft 61 by using the methods described above with anappropriate transducer assembly 62 which includes a magnetisedtransducer region 64 of shaft 61, the motor induced longitudinalmagnetic flux 60 present in the transducer region 64 of drive shaft 61can generate large sensor offset signals. The drive shaft itselfprovides the magnetic sensor host for the transducer region. Theseoffset signals are modulated by the changes of the mechanical load onthe motor axis and the supplied electrical current to the motor. Theoffset is therefore dynamic and cannot be easily compensated for.

A solution to the problem explained with reference to FIG. 6 is seen inFIG. 7. A pair of coils L1 and L2 are axially spaced about transducerregion 64 and they are energised to provide a longitudinal magneticfield in region 64 that counteracts the field due to the motor 63.

As indicated in FIG. 7, the level of the interfering magnetic fieldstrength may be measured in real time by an axially oriented sensor(such as sensor 7 or 21) forming part of the transducer assembly 62 andcontrolling a compensating current source 65 that energises coils L1 andL2 connected in series with a current I of a magnitude to cancel themotor induced field 60.

To allow measurement of longitudinal (axially-directed) field componentsof the transducer region, the compensating action can be set up under notorque conditions for circumferential magnetisation, then held at thatvalue. Otherwise the adjustment can be done manually to establish apreset current value. The technique most suitable will depend on thecircumstances of each individual installation.

FIGS. 8a and 8 b illustrate a preferred implementation of the activecompensation technique of FIG. 7. These figures show a collar structurewhich finds application in various other embodiments of the inventiondescribed below. In FIGS. 8a and 8 b the shaft 61 is collared at 20 toproduce a recess 25 the base of which extends about transducer region 64and which aids in causing internal longitudinal flux to “leak”externally to the shaft and be detectable. The external longitudinalflux is detected by sensors 24 which may be in controlling a currentgenerating means for energising coils L1, L2 to counteract the externallongitudinal flux as previously described and/or as part of the torquemeasurement process. If the region 64 is longitudinally magnetisedtorque is measured using sensor 23 (preferably a pair of diametricallyopposed sensors) to detect a torque-dependent component of the externalflux. In FIG. 7 the transducer region 64 lies between coils L1 and L2within the sensor arrangement which is adjacent to but does not contactthe shaft. Similarly in FIGS. 8a and 8 b the magnetised transducerregion is located in the region forming the base of recess 25 withnon-contacting sensors 23 and 24. The collar structure is applicable toa transducer region circumferentially or longitudinally magnetised. InFIGS. 8a and 8 b the sensor arrangement is appropriate to longitudinalmagnetisation.

FIGS. 9 and 9b show an arrangement similar to that of FIGS. 7 and 8 inthat it seeks to back off or nullify the motor leakage flux in shaft 61.It is intended for higher levels of flux. L1 and L2 are energised asbefore, for example in dependence on the flux sensed at 24. A housing 70of magnetic material providing a magnetic shield encloses the transducerregion 64 of the shaft 61 and the adjoining coils L1, L2. The shield 70is apertured at 72 a and 72 b for passage of the shaft and theseapertures provide axially-spaced, magnetic poles of opposite polaritybetween which the collared region 64 is located. The poles act on theshaft 61 to induce a longitudinal flux through the transducer region 64to counteract longitudinal flux in the shaft due to the driving motor.The poles 72 a, 72 b are magnetised by one or more coils wound about thehousing 70. Specifically a pair of coils L3 and L4 are shown and L3/L4are energised by current I′ dependent on the flux sensed by 24. Thepoles concentrate the shield flux. The polarity induced is the same asthe coils L1 and L2. For a small shaft diameter the magnetic shield andcoil L3/L4 structure enables higher ampere turn ratings to beaccommodated for large leakage fluxes. The combination of L3/L4 andshield on one hand and the coils L1/L2 on the other may be appliedseparately. The shield arrangement may be advantageous when there areother stronger sources of stray magnetic field in the vicinity of thetransducer. For example the shield may protect the transducer fromfields of the order of 100 or more Gauss, whilst coils L1 and L2typically protect against fields of the order of tens of Gauss.

A different approach is adopted in the apparatus of FIGS. 10a, 10 b and11. Rather than nullifying the longitudinal flux from motor it isinstead used as the transducer flux source in a longitudinalmagnetisation type measurement. Here again a collared structure 20 aidsin outwardly deflecting the longitudinal flux in the region 64 forproducing a longitudinal (axial) directed external field. Thelongitudinal sensors 24 measure the longitudinal flux (of whatevervalue). The transverse sensor(s) 23 measures the circumferentialcomponent. The torque calculation is made independent of the actual fluxin the shaft by using this as a reference. The measurement from sensors24 is used as a reference against which the torque-dependent componentvalue from sensor(s) 23 is measured.

The axial component (measured by 24) is used to determine the maximumavailable field strength to measure torque at the sensor region. Theresult of this measurement is used to control the gain of processingcircuitry for providing a signal representing torque. The greater thelongitudinal magnetic field 60, the higher the sensitivity of themagnetic field measured by the circumferentially arranged magnetic fieldsensors. Therefore the amplification gain in the signal conditioningelectronics for the circumferentially magnetic field sensors need to bereduced in proportion to an increase in the longitudinal magnetic field.

As shown in FIG. 11, the longitudinal field 60 that extends through theregion 64 will be deflected as indicated at 60 a in relation to theapplied torque forces on the drive shaft 61. The whole shaft effectivelyacts as a force sensor. The greater the torque, the larger thecircumferential component of the field, measured by sensor 23.

In the embodiments of FIG. 7 and FIGS. 8a and 8 b, the current in coilsL1 and L2 is applied so that the loop fields compensate or nullify theinterfering field. A similar coil arrangement to that illustrated inFIG. 7 and in FIGS. 8a and 8 b can be used in a different way in atechnique which aims to eliminate the effect of the interfering fieldfrom the torque-sensing operation rather than cancelling or compensatingthe interfering field. This is illustrated in FIG. 12.

In FIG. 12 the coils L1 and L2 are not energised in dependence on asensed field but to the contrary are energised to create a fielddistinguishable from interfering fields. To this end the coils L1 and L2are connected to an AC source 30, preferably a pulse-type source, toinduce an alternating magnetic field in the transducer region 32 betweenthe coils. This is a longitudinal field. The source frequency shouldavoid a relationship with main supply frequencies (50 or 60 Hz) or anyother frequency imposed by the operation of the motor or machine withwhich the shaft is associated. Conveniently the source frequency is inthe audio range, say between 500 Hz and 10 kHz. A frequency around 1 kHzwould be suitable. It is also a frequency within the sensing capabilityof saturating-core type of sensors. Hall effect or magnetoresistivetypes of sensor may be expected to have a higher frequency response butfrequency limitations may also be imposed in driving the coils L1 andL2.

The alternating magnetic field provides an alternating torque-dependentcomponent at the source frequency sensed by the sensor(s) 23. The totaltorque-dependent component to which sensor(s) 23 responds may include aDC component from a machine-induced interference field or another ACcomponent associated with the main frequency or a frequency emanatingfrom the motor driving the shaft. The wanted source frequency componentis extracted from the unwanted noise components by a filter 34 feedingor included within signal-processing unit 38 from which the torquerepresenting signal T is obtained. The filter 34 may be realised inhardware or software and the filter frequency driven from the source asindicated by the chain line 36 to ensure the filter tracks the sourcefrequency. Synchronous detection in which the detector is drive by asignal from source 30 may be employed. All these techniques arewell-known.

The sensors (24) can be used to derive a reference signal for derivingthe torque from the torque-dependent component provided by sensor 23.The reference signal in this case is a component at the source frequencyand is subject to filtering at 31 in the same way as thetorque-dependent component is filtered. To this extent operation issimilar to that of the embodiment of FIGS. 10a, 10 b and 11.

Another approach to torque measurement is illustrated in FIGS. 12a and12 b. As the shaft 61 rotates a circumferential band 16 is cleaned by amagnetic erase head(s) 12 of the kind used in magnetic recording.Following the erase head (downstream), a write-head 13 writes a magnetictrack 15 (of any kind) of width w. The shaft should preferably berotating at at least 100 rpm when using this technique. The write-head13 is oriented to have the head gap transverse to the axis of rotationof the shaft and preferably perpendicular to the axis of rotation sothat the gap lies tangential or circumferentially disposed with respectto the rotating shaft surface.

The two read-heads 14 a and 14 b are spaced relative to the width w togive no signal when the shaft is barely rotating or known balancedsignals that can be nulled. As torque builds in the shaft it has beenfound that the signals from the read-heads 14 a and 14 b becomeunbalanced to an extent dependent on the value of the torque. Thisreaction to torque is as if the magnetised track 15 or the fluxassociated with it is slightly deflected one-way or the other dependenton direction of rotation to produce an unbalance output from theread-heads 14 a and 14 b that is a measure of torque.

The write-head 13 may preferably modulate the track 15 in some way toprovide a signal at each read head that can be separated from noise. Tothis end the write-head can be energised with a pulse waveform at agiven frequency.

Filtering at the source frequency is applied to the read-heads 14 a and14 b. This frequency-selective mode of operation is similar to thatdescribed for the embodiment of FIG. 12. The read pulses in FIG. 13 willbe delayed with respect to the write pulses to an extent which is usableas a measure of the rate of rotation.

What is claimed is:
 1. A torque transducer for measuring torque in arotating shaft of the kind having a transducer region, for example aregion storing a permanent magnetisation, in which a magnetic transducerfield is established and at least one non-contacting sensor adjacent thetransducer region to develop a torque-dependent signal, wherein inoperation the shaft is subject to longitudinal flux generated by meansexternal to the transducer region, characterised by a non-contactingsensor responsive to a component of said longitudinal flux to develop asignal representing the level of said longitudinal flux, and meansresponsive to the level-representing signal for said longitudinal fluxand magnetically coupled to said shaft to generate a compensating fluxto counteract said longitudinal flux at the transducer region.
 2. Atorque transducer as claimed in claim 1 wherein said means forgenerating the compensating flux comprises at least one current-carryingcoil about the shaft to be magnetically coupled thereto.
 3. A torquetransducer as claimed in claim 1 said means for generating thecompensating flux comprises a magnetic structure having poles spacedalong the shaft and at least one current-carrying coil wound on saidmagnetic structure.
 4. A torque transducer as claimed in claim 1 inwhich said shaft carries a collar structure comprising twoaxially-spaced portions in the space between which is disposed thesensor responsive to the component of longitudinal flux.
 5. A torquetransducer for measuring the torque in a rotating shaft which, inoperation, has a longitudinal field extending therealong, wherein atleast one sensor is placed in non-contacting fashion adjacent a portionof the shaft to sense and provide a signal dependent on a transversecomponent of flux arising from the longitudinal flux due to the torquein the shaft.
 6. A torque transducer as claimed in claim 5 in which afurther non-contacting sensor is mounted to sense the longitudinal fluxto provide a reference signal.
 7. A torque transducer for a rotatingshaft comprising flux generating means for generating a magnetic fluxextending longitudinally in a portion of the shaft, said flux generatingmeans being magnetically coupled to said shaft at axially spacedlocations between which said portion is situated, at least one sensorplaced in non-contacting fashion adjacent said portion to provide asignal dependent on a transverse component of flux arising from thelongitudinal flux in said portion due to the torque in the shaft, saidmagnetic flux generating means being operable to generate an alternatingmagnetic field at a selected frequency, and said at least one sensorsignal being processed by frequency selective means operable at saidselected frequency to provide a signal representing torque in the shaftderived from said alternating magnetic field.
 8. A torque transducer asclaimed in claim 7 in which said shaft transmits in operation anotherlongitudinal flux, not generated by said flux generating means saidselected frequency enabling the signal dependent on the transversecomponent of flux to be separated from any signal due to said otherlongitudinal flux in processing by said frequency selective means.
 9. Atorque transducer as claimed in claim 8 in which said flux generatingmeans operates in a pulsed mode.
 10. A torque transducer element asclaimed in claim 7 in which said flux generating means comprises a pairof spaced coils wound about said shaft and between which said portion issituated and means for energising said coils at the selected frequency.